The invention is directed to the field of cordless telephony, and in particular to the field of digital cordless telephony.
Cordless telephones comprise a base unit and a handset, wherein the base unit is typically coupled to a telephone network, such as the public switched telephone network, and the handset is free to roam within the vicinity of the base unit. The area within which the handset can roam is limited by the range of an RF link between the base unit and the handset.
The early versions of cordless telephones were known as xe2x80x9canalogxe2x80x9d cordless telephones, and typically operated in the 46-49 MHz range. The xe2x80x9canalogxe2x80x9d designation of these telephones derives from the analog nature of the RF link. The analog RF link is a product of transceivers in each of the base unit and the handset which would simply play an audio stream, as a radio broadcaster, for reception by the matching transceiver. This radio transmission could be picked up by any other receiver that was set to the transmitted frequency, such as by a scanner, or by a baby monitor.
Some high end versions of these analog telephones include an answering machine in the base unit. The answering machine typically includes a digital memory within which encoded speech messages are stored. An advantageous feature of such a high end analog cordless telephone is the ability of a user to monitor answering machine messages from the handset. Upon the appropriate signals provided by the handset to the base unit in response to user activation of a handset keypad, the base unit decodes stored speech messages, converts the decoded speech messages to analog signals via a digital-to-analog converter, and the analog speech is transmitted via the RF link. The user can thus listen to the stored messages from the handset.
More recent versions of cordless telephones are known as xe2x80x9cdigitalxe2x80x9d cordless telephones because the RF link is a digital link. When a user is engaged in a telephone call, the user""s voice is converted in the handset to Adaptive Differential Pulse Code Modulated (ADPCM) digital samples, which are transmitted to the base unit in real time, while incoming voice signals from a distant party to a conversation are received by the base unit from a telephone network and converted to ADPCM digital samples, which are transmitted from the base unit to the handset. An advantage of a digital cordless telephone is that it is more secure, since the digital RF transmission is not as easily monitored. Also, the digital RF link tends to be more consistent and to have a greater range than an analog cordless telephone.
However, a drawback of a digital cordless telephone is that it is not necessarily compatible with the feature of monitoring stored messages from the handset, because the quality of the message is seriously degraded if the decoded message is ADPCM encoded by the base unit and then ADPCM decoded in the handset. There is a need, therefore, for a digital cordless telephone that supports the monitoring of stored messages from the handset, while maintaining a high voice quality.
This need is met, in one embodiment according to the invention, by a digital cordless telephone including a base unit adapted to couple to a telephone network, to transmit an outgoing message to an incoming caller, to store a digital message from the incoming caller, and to transmit and receive digital samples over an RF link. The digital cordless telephone of this embodiment also includes a handset adapted to transmit and receive digital samples over the RF link. According to this embodiment, the base unit and handset are further adapted to enable a user to monitor the stored digital message from the handset.
In an alternative embodiment according to the invention, a method of accessing a digital message stored in one of a base unit and a handset of a cordless telephone from the other of the base unit and the handset includes the steps of transmitting a control signal from the accessing device to the storing device, retrieving the digital message, in encoded form, from a memory in the storing device, and transmitting at least a portion thereof to the accessing device. The method further includes the step of decoding the encoded digital message in the accessing device.